This invention relates generally to communication systems using polar modulation techniques. More particularly, this invention relates to a method and apparatus for generating linearly modulated signals using polar modulation in remote stations in a cellular communication system.
Modern communication systems, such as cellular and satellite radio systems, employ various modes of operation (analog, digital, dual mode, etc.), and access techniques such as frequency division multiple access (FDMA), time division multiple access (TDMA), code division multiple access (CDMA), and hybrids of these techniques.
FIG. 1A is a block diagram of an exemplary cellular mobile radiotelephone system, including an exemplary base station 110 and mobile station 120. The base station includes a control and processing unit 130 which is connected to a mobile switching center (MSC) 140 which in turn is connected to the public switched telephone network (PSTN) (not shown). General aspects of such cellular radiotelephone systems are known in the art. The base station 110 handles a plurality of voice channels through a voice channel transceiver 150, which is controlled by the control and processing unit 130. Also, each base station includes a control channel transceiver 160, which may be capable of handling more than one control channel. The control channel transceiver 160 is controlled by the control and processing unit 130. The control channel transceiver 160 broadcasts control information over the control channel of the base station or cell to mobiles locked to that control channel. It will be understood that the transceivers 150 and 160 can be implemented as a single device, like the voice and control transceiver 170, for use with control and traffic channels that share the same radio carrier.
In a typical transceiver, such as the mobile station 120, baseband communication signals are phase, frequency, or amplitude modulated on a carrier signal, and the modulated signal is transmitted from the transceiver.
There are various modulation techniques. Quadrature Amplitude Modulation (QAM) and Phase Shift Keying (PSK) are examples of linear modulation techniques. These techniques typically employ quadrature (I-Q) amplitude modulators, as illustrated in FIG. 1B. Based on the information 100 to be transmitted, a digital signal processor 101 generates digital in-phase (I) and quadrature (Q) components. These digital I and Q components are then converted to analog signals using D/A-converters 102a,b. The analog signals are low pass filtered in low pass filters 103a,b. The output of the filters 103a,b respectively modulate, using multipliers 104a,b, carrier signals 105a,b that are separated 90.degree. in phase. The outputs of the multipliers 104a,b are summed in an adder 106 to form a signal 107 to be amplified in a power amplifier and transmitted.
Another modulation technique, which is often considered more efficient, is polar modulation. This technique is based on a polar representation of the baseband signal, as disclosed for example in U.S. Pat. No. 5,430,416 and illustrated in FIG. 2. According to this technique, polar components, i.e., amplitude (r) and phase ((p) components, are used instead of I and Q components. Based on the information 200 to be transmitted, a digital signal processor 201 generates an amplitude component 202 and a phase component 203. The phase component 203 modulates the carrier signal in a phase modulator 205, resulting in a phase modulation with constant envelope. The amplitude component 202 is converted to an analog signal in a D/A-converter 204 and then fed through a regulator 206 which adjusts the current or voltage of the signal controlling the power of a power amplifier 207 to a target power value based on the analog signal and the output signal 208. The regulated analog signal modulates the phase modulated carrier signal in the power amplifier 207 by controlling the power of the power amplifier. The resulting amplified signal 208 is then output for transmission.
To modulate the phase modulated signal with the amplitude component, it is necessary to have the amplitude component phase aligned with the phase transitions experienced by the carrier signal. In conventional polar modulation devices, phase distortions are created in components such as the power amplifier, resulting in a mis-alignment between the amplitude component and the phase modulated carrier signal. The resulting amplitude and phase modulated carrier signal will thus differ from the desired modulated carrier signal.
Various attempts have been made to solve this problem, including a technique disclosed in U.S. Pat. No. 4,972,440. According to this patent, the phase distortion introduced in the power amplifier is compensated for by applying a corresponding phase distortion to the carrier signal.
In addition to the phase distortions created by the power amplifier, there are also phase distortions created by the phase modulator which may contain components that function substantially as a low pass filter. The phase modulator is often assumed to be ideal, i.e., it is assumed that the phase modulator does not create a phase distortion. However, in actual applications, the phase modulator is not ideal and it produces a phase distortion causing a mis-alignment between the amplitude component and the phase modulated carrier signal. This may result in a distorted output signal having a frequency spectrum that is much wider than the frequency spectrum of the desired output signal. Due to this wide spectrum of the distorted output signal, the spectrum requirements for transmitting the signal may be unfulfilled. The known techniques for compensating for the phase distortions created by the power amplifier do not address the problem of phase distortions created by the phase modulator.
Thus, there is a need for a method and apparatus for compensating for phase distortions created by phase modulation in a polar modulation system.